1. Field of the Invention
The present invention relates to a laminated packaging material for a configurationally stable, liquid-tight packaging container possessing superior oxygen gas barrier properties, the packaging material having a configurationally rigid, but foldable core layer of paper or paperboard, and a layer of polyvinyl alcohol applied as oxygen gas barrier on one side of the core layer. The present invention also relates to a method of producing the laminated packaging material, as well as configurationally stable, liquid-tight packaging containers possessing superior oxygen gas barrier properties produced from the laminated packaging material.
2. Description of the Related Art
Within the packaging industry, liquid foods are nowadays often packed and transported in consumer packages of the single-use type, and a large group of these so-called disposable packages is produced from a laminated packaging, material (packaging, laminate) of the type which has a mechanically stable core layer of paper or paperboard, and outer liquid-tight coatings of plastic, preferably polyethylene, applied on both sides of the core layer.
The point of departure for the composition of the laminated packaging material is to afford the packed product the best possible production protection properties, at the same time as it must be easy to produce, from such packaging material, packaging containers which in turn must be both convenient and easy to handle (`consumer friendly`).
A conventional packaging laminate consisting solely of paper or paperboard and plastic, as described above, makes possible the production of configurationally stable, liquid-tight packaging containers but lacks oxygen gas barrier properties and cannot therefore be used for packaging and transporting oxygen gas sensitive products without first being modified or supplemented.
From, for example, Swedish Patent No. 440 519, it is previously known to supplement the above-described plastic coated paper or paperboard material with the desired oxygen gas barrier properties by incorporating into the laminated packaging material a layer of polyvinyl alcohol which constitutes but one of numerous other known examples of so-called barrier materials possessing superior tightness properties vis-a-vis oxygen gas.
According to Swedish Patent No. 440 519, the supplementary polyvinyl alcohol layer is applied by coating or other form of covering operation of an aqueous emulsion of polyvinyl alcohol on one side of a paper or paperboard web which thereafter, together with the applied aqueous polyvinyl alcohol layer, is dried so as to adjust the moisture content of the polyvinyl alcohol layer (and thereby its degree of efficiency as an oxygen gas barrier). Finally, the thus moisture content adjusted polyvinyl alcohol layer is covered with a thin, liquid-tight thermoplastic coating, preferably polyethylene, which is extruded on the polyvinyl alcohol layer for the purpose of protecting the polyvinyl alcohol layer against the penetration of moisture and wet which would drastically weaken the oxygen gas barrier properties of the polyvinyl alcohol layer. Liquid-tight plastic coatings of polyethylene moreover possess the advantage that they render the thus produced packaging material sealable by conventional heat sealing.
In order to ensure that the applied aqueous polyvinyl alcohol layer, after drying, forms a continuous polyvinyl alcohol layer on the paper or paperboard web, the aqueous polyvinyl alcohol emulsion should, according to Swedish Patent No. 440 519, also contain a thickening agent which aids in increasing the viscosity of the aqueous emulsion and thereby counteracts the tendency of the aqueous emulsion to penetrate into the liquid-absorbent paper or paperboard substrate.
While the problem of penetration of polyvinyl alcohol into the absorbent paper or paperboard layer may be solved to some degree by the viscosity-increasing addition of a thickening agent to the aqueous polyvinyl alcohol emulsion, as proposed in Swedish Patent No. 440 519, it is nevertheless necessary that the aqueous polyvinyl alcohol emulsion be applied in an excessively large coating quantity in order to ensure the integrity of the dried polyvinyl alcohol layer.
The problem of excessive coating quantities as required in the prior art technique is partly linked to the fact that the paper or paperboard web is not entirely smooth and even but has an irregular and uneven surface structure (or coarseness) which must be "filled out" in order to form the desired smooth substrate surface for the polyvinyl alcohol emulsion.
Alternatively, this problem may be avoided by employing a smoother and more even paper or paperboard quality as starting material, but such qualities are much more expensive than conventional packaging qualities, and would render production of the laminated packaging material considerably more expensive.